This application is based upon and claims the benefit of Japanese Patent Applications No. 10-17992, filed on Jan. 14, 1998, and No. 10-365261 filed on Dec. 22, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a wave soldering method for soldering on a reflow side of a work-piece by jetting molten solder onto the reflow side, and a system used for the wave soldering method.
2. Description of the Related Art
Conventionally, a printed circuit board is soldered by a wave soldering method to improve processing efficiency. A wave soldering system 9 coventionally used for the wave soldering method is shown in FIG. 11. The wave soldering system 9 includes two solder baths 93, 94. A flux coating device 91 for coating flux onto a reflow side of a printed circuit board 8 and a pre-heater 92 for heating the printed circuit boad 8 are disposed on an upstream side of the solder bath 93. A transfer unit 93 is disposed above these parts to sequentially transfer the printed circuit board 8.
When a soldering process is carried out by the wave soldering system 9, the printed circuit board 8 is sequentially transferred by the transfer unit 95 to pass through above the flux coating device 91, the pre-heater 92, and the solder baths 93, 94. Accordingly, the back face (reflow side) of the printed circuit board 8 is soldered. Specifically, in the solder baths 93, 94, as shown in FIG. 12, molten solder 7 is jetted upward from a wave nozzle 930 which is disposed within the solder bath 93, using a solder circulating unit 97. The printed circuit board 8 is transferred with the reflow side 81 contacting the molten solder 7.
In the conventional method and system described above, however, the printed circuit board 8 can be warped by heat transmitted from the molten solder 7. When the printed circuit board 8 is thermally warped, the following deficiencies arise. First, as shown in FIG. 13, when the printed circuit board 8 is largely warped by the heat, the position of the printed circuit board 8 where the molten solder 7 is jetted thereon may be go down. In this case, an immersion depth of the printed circuit board 8 into the molten solder 7 is unnecessarily increased. Accordingly, bridges, solder short-circuitted parts, and the like are formed on the printed circuit board 8, and as shown in FIG. 14, solder climbing deficiency such that the molten solder 7 reaches the surface of the printed circuit board 8 on an opposite side of the reflow side from its side portion arises. Further, when the printed circuit board 8 is warped so that a circumference portion thereof escapes upward as shown in FIG. 15, the circumference portion may not contact the molten solder 7, thereby causing a non-soldered portion of the printed circuit board 8.
Various types of wave soldering methods and systems have been proposed to stably perform wave soldering. For example, JP-A-7-131143 discloses a method for stabilizing a soldered sate by accurately detecting a height of jetted molten solder. This kind of method for controlling a molten solder level is also disclosed in JP-A-2-37964.
However, the deficiencies caused by thermal deformation of the printed circuit board itself cannot be solved by these conventional methods. Therefore, even when the height of the jetted molten solder is precisely controlled, it is difficult to prevent the solder climbing deficiency and the occurrence of non-soldered portion.
The present invention has been made in view of the above problems. An object of the present invention is to provide a wave soldering method for soldering on a work-piece and a wave soldering system used for the method capable of performing a soldering process with high quality even when the work-piece is thermally warped.
According to the present invention, in a method for soldering on a reflow side of a work-piece by bringing the work-piece into contact with a wave molten solder, a soldering process is performed while controlling an immersion depth of the work-piece into the wave molten solder to be constant. The immersion depth of the work-piece can be controlled by adjusting one of a height position of the work-piece and a wave height of the wave molten solder based on a displacement amount of the work-piece. Accordingly, even when the work-piece is thermally warped, the immersion depth can be controlled to be constant. As a result, the soldering process can be performed with high quality.
The displacement amount can be detected by a displacement sensor serving as a displacement device. When the displacement sensor is disposed above a contact portion between the work-piece and the wave molten solder, the immersion depth can be detected directly by the displacement amount. Therefore, the one of the height position of the work-piece and the wave height of the wave molten solder can be controlled in response to the displacement amount. When the displacement amount is detected at a plurality of points of the work-piece, a warp amount of the work-piece is computed based on the displacement amount and the one of the height position of the work-piece and the wave height of the wave molten solder is controlled based on the warp amount.
In these case, the wave height of the wave molten solder can be directly detected by a wave height detecting unit. At that time, the one of the height position of the work-piece and the wave height of the wave molten solder is controlled based on the displacement amount of the work-piece and the wave height detected by the wave height detecting unit. Accordingly, the immersion depth control can be performed more precisely. Further, even when the wave height of the wave molten solder fluctuates, the immersion depth control can quickly response to the fluctuation.
The displacement amount can be detected using a light emitting device which is disposed on a side of a contact portion between the work-piece and the wave molten solder for emitting a measurement light onto a first irradiation position of one of the wave molten solder and the work-piece. In this case, the measurement light is reflected at the first irradiation position and then is reflected a second irradiation position of another one of the work-piece and the wave molten solder as a reflected light. The one of the height position of the work-piece and the wave height of the wave molten solder can be controlled so that an optical path of the reflected light becomes constnat. Otherwise, the one of the height position of the work-piece and the wave height of the wave molten solder can be controlled so that an interval between the first and second irradiation positions becomes constant.
In this case, likewise, the wave height can be directly detected by the wave height detecting unit so that the immersion depth control is performed more precisely. It is preferable that the measurement light is a laser having favorable directivity, resulting in accurate immersion depth control. It is preferably that the measurement is a pulsed light. Accordingly, a change in the optical path of the reflected light or the like can be stepwise detected, resulting in high responsibility characteristics of the immersion depth control.